The application of patient monitoring has been extended into a variety of settings, including traditional Intensive Care Units (ICUs), step-down and medium-care units, medical and surgical care floors, and at home. In many of these settings it is critical to have knowledge of the circumstances under which a set of vital signs has been obtained. One crucial piece of information is the body posture of the monitored user (for example, upright, supine, prone, laying on left side, laying on right side, etc) at the time of monitoring.
In clinical practice, there are many care protocols in place that require patient position to be accurately tracked and recorded in time. Some examples include the protocols of recording vital signs, interpreting Screen TrendsInnovative (ST) maps, preventing pressure ulcers, detecting bed exit, detecting a fall, weaning from mechanical ventilation, pneumonia prevention, etc. Nowadays, miniature patient monitoring devices are available which contain accelerometers that can measure vital signs (such as respiration and pulse rate) and that can, at the same time, measure the inclination of the device based on an observation of gravity. From the sensor signals obtained from these devices and the knowledge of the location and/or orientation of the device with respect to the body of the monitored user, the posture of the user can be estimated and tracked.
When a device is placed on or attached to a user, it is desirable for the measurement reference frame of the accelerometer in the device to be aligned with the reference frame of the body of the user. However, due to the variability in body shapes of users and individual device attachment, there is likely to be some misalignment between these reference frames. To accurately monitor the posture of a user, it is necessary to determine the relative orientation between the measurement reference frame and the reference frame of the body of the user. This relative orientation is illustrated in FIG. 1.
In FIG. 1, zb represents the z-axis of the reference frame of the user which is aligned normal to the user's body (i.e. it is perpendicular to the plane of the user's back, directed towards the front of the body), yb represents the y-axis of the reference frame of the user which is aligned substantially vertically upwards towards the head of the user, and xb represents the x-axis of the reference frame of the user which is orthogonal to the y- and z-axes. Although the device has been placed on or attached to the user in an attempt to align the measurement reference frame of the accelerometer (represented by axes xa, ya and za) with the reference frame of the user, it can be seen that, due to the shape of the part of the body to which the device has been attached, the device may have undergone rotation, resulting in a difference or misalignment between the reference frames.
A calibration procedure is therefore required in order to compute this unknown relative orientation, and to allow for accurate posture estimation.
Previously proposed methods for calibration (such as that disclosed in U.S. Pat. No. 6,044,297) require a user to adopt multiple postures in order for an initial state to be recorded. However, in practice, it is often not desirable or even feasible to ask for the cooperation of a user to adopt such a sequence of postures during the calibration procedure because the user may be too unwell, required to remain still for medical reasons, unconscious, etc. Ideally, it should be possible to perform a calibration procedure without any active involvement or support from the user.